In recent years, the miniaturization technology for electronic devices has been rapidly developed, and various kinds of portable electronic devices are becoming popular. Also, a battery, which is a power supply for these portable electronic devices, has been required to be miniaturized, and a nonaqueous electrolyte secondary battery having high energy density is attracting attention.
The nonaqueous electrolyte secondary battery obtained by using metallic lithium as a negative electrode active material is characterized in that the battery life is short because a dendritic crystal called dendrite precipitates on a negative electrode during charge although energy density is very high. Also, in this nonaqueous electrolyte secondary battery, dendrite can be grown so as to reach a positive electrode, thereby causing an internal short circuit, and there are problems in safety. Therefore, a carbon material capable of absorbing and desorbing lithium, specifically graphitic carbon, has been used as a negative electrode active material substituted for metallic lithium.
In order to increase the energy density of a nonaqueous electrolyte secondary battery, it has been attempted to use materials having large lithium storage capacity and high density for a negative electrode active material. Examples of such materials include an amorphous chalcogen compound and elements such as silicon and tin which form an alloy with lithium. Among these materials, silicon can absorb lithium until the atomic ratio Li/Si of lithium atoms to silicon atoms reaches 4.4. Thus, the negative electrode capacity per mass of the negative electrode active material (silicon) is about 10 times as large as that of graphitic carbon.
Silicon is characterized in that the volume thereof largely changes associated with the insertion and desorption of lithium in charge and discharge cycle. The volume of a negative electrode also changes associated with the volume change of silicon, and an internal short circuit occurs in a nonaqueous electrolyte secondary battery. Consequently, a nonaqueous electrolyte secondary battery can be in an overdischarged state which is beyond a usual charge and discharge range. When a nonaqueous electrolyte secondary battery is in an overdischarged state, the resistance of an electrode is increased, which causes an excessive electrification and a voltage drop. For these reasons, the temperature of a nonaqueous electrolyte secondary battery is increased, which causes the reduction in cycle life.